Gallium nitride (GaN) is a wide-bandgap semiconductor material widely known for its usefulness as an active layer in blue light emitting diodes. GaN is also under investigation for use in other microelectronic devices including laser diodes and high-speed, high power transistor devices. As used herein, “gallium nitride” or “GaN” refers to gallium nitride and III-nitride alloys thereof, including aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and aluminum indium gallium nitride (AlInGaN).
High-quality bulk crystals of GaN are currently unavailable for commercial use. Thus, GaN crystals are typically fabricated as heteroepitaxial layers on underlying non-GaN substrates. Unfortunately, GaN has a considerable lattice mismatch with most suitable substrate crystals. For example, GaN has a 15% lattice mismatch with sapphire and a 3.5% lattice mismatch with silicon carbide. Lattice mismatches between a substrate and an epitaxial layer cause threading dislocations which may propagate through the growing epitaxial layer. Even when grown on silicon carbide with an aluminum nitride buffer layer, a GaN epitaxial layer exhibits dislocation densities estimated to be in excess of 108/cm2. Such defect densities limit the usefulness of GaN in highly sensitive electronic devices such as laser diodes.
Lateral Epitaxial Overgrowth (LEO) of GaN has been the subject of considerable interest since it was first introduced as a method of reducing the dislocation densities of epitaxially grown GaN films. Essentially, the technique consists of masking an underlying layer of GaN with a mask having a pattern of openings and growing the GaN up through and laterally onto the mask. It was found that the portion of the GaN layer grown laterally over the mask exhibits a much lower dislocation density than the underlying GaN layer or the portion of the GaN layer above the mask openings. As used herein, “lateral” or “horizontal” refers to a direction generally parallel to the surface of a substrate, while the term “vertical” means a direction generally orthogonal to the surface of a substrate.
One drawback to conventional LEO techniques is that separate process steps are required for growing the underlying GaN layer, masking the GaN layer and then growing the lateral layer. Early embodiments of LEO did not place the mask directly on the non-GaN substrate because unwanted nucleation would occur on the mask during nucleation of the GaN layer at low temperatures, preventing adjacent laterally-grown regions from coalescing (or otherwise from growing laterally a desired distance if coalescence is not required). When the mask is placed directly on a GaN layer, unwanted nucleation on the mask is typically not a problem since low temperature nucleation is not required and the growth temperature of GaN is very high, typically above 1000° C. During high temperature growth, unwanted nucleation does not occur on the mask due to the much higher sticking coefficient of gallium atoms on the gallium nitride surface as compared to the mask.
This drawback is addressed with some success by a “single step” process for LEO. Shealy et al. disclosed a process whereby an underlying SiC or sapphire substrate was masked with silicon nitride. The process is referred to as “single step” because it does not require growth of an intermediate layer of GaN between the substrate and the mask. Shealy found that minimizing nucleation on the silicon nitride mask permitted growth of a relatively defect free layer of laterally-grown GaN over the masks. However, under certain circumstances it is desirable to avoid having to minimize nucleation on the mask, yet still be able to grow a relatively defect free layer of GaN in a single step process.